The fall season is only a month old — but already the cracks are starting to show.

Numbers for “Pan Am,” ABC’s much-hyped ’60s-era glamour series about life aboard the now-defunct airline starring Kelli Garner, plummeted precipitously Sunday night in its third episode.

The show’s numbers fell to 6.4 million viewers — a drop of over 4 million viewers from the show’s Sept. 25 premiere (11 million) and down nearly 2 million from last week.

More alarmingly, “Pan Am” was down 27 percent from the week before in adults 18-49 — the group most coveted by advertisers — which doesn’t bode well for the future.

Retro series have struggled so far this season. NBC canceled “The Playboy Club” — another ’60s-era show — after only three episodes when the show failed to attract viewers.

On the plus side for “Pan Am,” it did beat CBS’s combination of “The Good Wife” and “CSI: Miami” from 10 to 11 p.m. in adults 18-34 (but lost by a wide margin in total viewers).

“Pan Am” was hindered, in part, by its lead-in, “Desperate Housewives,” which averaged 8.4 million viewers to tie a series low. Both ABC and CBS were beaten for the night by NBC’s “Sunday Night Football” game, which averaged around 20 million viewers, according to Nielsen.